Overvoltage protection for SMPS based on demagnetization signal

ABSTRACT

An overvoltage protection circuit is provided for a switched-mode power supply of the type having a transformer with a primary winding, an auxiliary winding, and a secondary winding, with a switching transistor coupled in series with the primary winding. The overvoltage protection circuit includes a sensing resistor having a first terminal directly connected to the auxiliary winding for generating a current proportional to the voltage across the auxiliary winding, with a second terminal of this resistor being coupled to an input of a current mirror. The reflected current output of the current mirror is compared to an overvoltage reference current generated by an overvoltage reference current source by means of a current comparator, and a circuit is provided to generate a signal to turn off the switching transistor when the reflected current exceeds the overvoltage reference current. Such an overvoltage protection circuit provides fast, efficient and accurate overvoltage protection for the switched-mode power supply.

BACKGROUND OF THE INVENTION

The invention is in the field of power supply protection circuits, andrelates more particularly to an overvoltage protection circuit for aswitched-mode power supply.

Switched-mode power supplies are well known in the art, and aretypically used to obtain good regulation (due to switched feedback) aswell as efficient and economical filtering, since smaller-valuefiltering components can be used because of the typically higheroperating frequency of the switched-mode supply.

In switched-mode power supplies, it is desirable to provide overvoltageprotection to prevent the power supply output from exceeding apredetermined maximum value, thus preventing circuit malfunction andpossible damage to circuitry powered by the switched-mode power supply.

In prior-art overvoltage protection circuits, such as the overvoltageprotection circuit incorporated into the Motorola MC44603 power supplycontroller IC and illustrated in FIG. 44 on page 22 of the MotorolaTechnical Data Sheet for this device, a filtered DC voltage (V_(CC)) issensed by a resistive divider, then compared to a voltage referencelevel by a voltage comparator, the output of which is fed to a circuitwhich generates a signal that serves to turn off a switching transistorwithin the switched-mode power supply, thus causing the output voltageto drop below the overvoltage protection level. A similar circuit isshown in FIG. 1 of U.S. Pat. No. 5,313,381.

A prior-art overvoltage protection circuit of this general type is shownin simplified form in FIG. 1. In FIG. 1, an overvoltage protectioncircuit 10 senses an auxiliary power supply voltage V_(CC) which isgenerated from an AC voltage taken from an auxiliary winding 20 of apower supply transformer 21, which, in the simplified embodiment shown,is rectified by a diode 22 and filtered by a filter capacitor 24 toproduce the voltage V_(CC) at terminal 26. Voltage V_(CC) is sensed by aresistive divider composed of resistors 28 and 30, with a scaled-downvoltage proportional to V_(CC) being coupled to the noninverting (+)input of a voltage comparator 32. The inverting (-) input of voltagecomparator 32, at the terminal 34, is provided with a voltage referencesignal V_(REF). The output of voltage comparator 32, on line 36, isconnected to the Set input (S) of a circuit, such as the latch circuit38 shown here, for providing an overvoltage indicator signal V_(OVP) atthe output (Q) of the latch circuit at terminal 42. In order to restorethe power supply circuit to its normal operating condition, a resetsignal V_(RESET) is applied to Reset (R) terminal 40 of the latchcircuit 38.

The prior-art circuit of FIG. 1 operates by sensing the rectified,filtered voltage V_(CC), stepping this voltage down using a resistivevoltage divider composed of resistors 28 and 30, and feeding thisstepped-down voltage to the noninverting input of comparator 32. Theinverting input 34 of the comparator is provided with a voltagereference signal V_(REF), with the attenuation ratio of the resistivevoltage divider and the value of the V_(REF) selected such that thecomparator output 36 trips at a voltage which exceeds the nominal valueof V_(CC) by a selected amount. When the comparator 32 trips, latch 38is set and an overvoltage protection signal V_(OVP) is generated onterminal 42. This signal is provided to a switching transistor (such astransistor 40 in FIG. 1 of U.S. Pat. No. 5,313,381) which is coupled inseries with the primary winding of the switched-mode power supplytransformer to inhibit switching and thereby reduce the output voltageof the power supply. When the power supply output returns to anacceptable level, latch 38 is reset by a signal V_(RESET) at resetterminal 40 and the circuit returns to its normal mode of operation.

Although the circuit of FIG. 1 provides adequate overvoltage protection,it suffers from several drawbacks. Since this overvoltage protectioncircuit senses the voltage across capacitor 24, typically a largecapacitor which will charge and discharge relatively slowly, theovervoltage protection circuit will inherently be slow to recognize anovervoltage condition due to increased transformer voltage.Additionally, since V_(CC) can vary by as much as 30 percent from itsnominal value due to such factors as transformer winding ratiotolerances, the voltage fed to the noninverting input of voltagecomparator 32, which is a scaled-down version of V_(CC), can have asimilar percentage variation, thus rendering the circuit relativelyinaccurate.

Accordingly, it would be desirable to have an overvoltage protectioncircuit for a switched-mode power supply which is both faster and moreaccurate than existing overvoltage protection circuits.

SUMMARY OF THE INVENTION

It is therefor an object of the invention to provide an overvoltageprotection circuit for a switched-mode power supply which is capable ofoperating faster than prior-art circuits and which also offers improvedaccuracy with respect to prior-art circuits.

In accordance with the invention, these objects are achieved by a newovervoltage protection circuit for a switched-mode power supply in whicha sensing resistor is provided having a first terminal directlyconnected to the auxiliary winding for generating a current proportionalto the voltage across the auxiliary winding, with a second terminal ofthe sensing resistor being coupled to an input of a current mirror. Thereflected current output of the current mirror is compared to anovervoltage reference current generated by an overvoltage referencecurrent source by means of current comparator, with a circuit beingprovided to generate a signal to turn off a switching transistor withinthe switched-mode power supply when the reflected current exceeds theovervoltage reference current.

In a preferred embodiment of the invention, the sensing resistor is avariable resistor, so that current generated therein can be adjusted tocompensate for variations due to such factors as transformer windingratio tolerances.

In a further preferred embodiment of the invention, the current mirroris provided with a delay circuit so that the overvoltage protectioncircuit will not be overly sensitive to a highly transient overvoltagecondition.

In yet a further preferred embodiment of the invention, the overvoltagereference current source provides an overvoltage reference current whichexceeds a nominal current in the sensing resistor by a selectedpercentage which is approximately equal to the percentage above anominal voltage of the switched-mode power supply at which the switchingtransistor is to be turned off.

An overvoltage protection circuit in accordance with the presentinvention offers a significant improvement in that faster, moreefficient and more accurate overvoltage protection is provided for theswitched-mode power supply in which such an overvoltage protectioncircuit is provided.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWING

The invention may be more completely understood with reference to thefollowing description, to be read in conjunction with the accompanyingdrawing, in which:

FIG. 1 shows a simplified schematic diagram of a prior-art overvoltageprotection circuit; and

FIG. 2 shows a simplified schematic diagram of a switch-mode powersupply incorporating an improved overvoltage protection circuit inaccordance with the invention.

In the drawing, like components are generally designated by likereference numerals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a simplified schematic diagram of a typical prior-artovervoltage protection circuit, as described above. Since this circuitoperates by sensing the rectified, filtered voltage on capacitor 24,circuit operation will inherently be slow, thus not affording optimumovervoltage protection and possibly resulting in circuit malfunction oreven permanent damage to sensitive circuit components. Additionally, asnoted above, since the voltage across capacitor 24 can varysubstantially from its nominal value, the prior-art overvoltageprotection circuit, which operates off a fixed-ratio scaled-down versionof the capacitor voltage, can have a relatively inaccurate overvoltagetrigger level.

These disadvantages are overcome by the overvoltage protection circuitof the present invention, which is shown in a simplified switched-modepower supply 50 in FIG. 2. In the circuit of FIG. 2, an AC input voltageV_(AC) is applied to a conventional diode bridge rectifier 52, theoutput of which is coupled to a filter capacitor 54 and a primarywinding 56 of transformer 21. The primary winding 56 is coupled toground through a switching transistor 64 and, optionally, a resistor 66.A secondary winding 58 of transformer 21 has its output coupled througha diode 60 to a filter capacitor 62, with the D.C. output voltage V_(DC)of the circuit being taken across this capacitor. Auxiliary winding 20of the transformer 21 is connected to diode 22 and filter capacitor 24to generate a D.C. voltage V_(CC) on terminal 26, which voltage can beused to power the control circuitry of the power supply. The portions ofthe circuit so far described are essentially similar to those of FIG. 1of U.S. Pat. No. 5,313,381, in both configuration and operation, andaccordingly will not be described in further detail.

Unlike the prior-art overall voltage protection circuits previouslydescribed, in the overvoltage protection circuit 70 of the presentinvention, as shown in FIG. 2, the protection circuit input is takendirectly from the auxiliary winding 20 of the transformer 21, atjunction 72, rather than from terminal 26, at which terminal theauxiliary winding output has been rectified and filtered. This input tothe overvoltage protection circuit is coupled through a sensing resistor74, here a variable resistor, to the input terminal 76 of a currentmirror 78 in order to provide an input current I_(SENSE) proportional tothe voltage at junction 72 to the input terminal 76 of the currentmirror 78. The current mirror 78 includes two MOSFET transistors 80 and82 in a conventional current mirror configuration. The current mirror 78may additionally include a delay circuit, in the form of an RC filtercomposed of resistor 84 and capacitor 86, which may optionally beincluded for the purpose of avoiding a false overvoltage protectioncircuit trigger condition due to a transient noise signal of shortduration. In practice, the values of resistor 84 and capacitor 86 may beselected in a known manner to provide a delay of about 500 nanoseconds.

The output of current mirror 78 is connected to a current source 88,which is coupled to a power line 90 and which provides a current ofn•I_(REF) A current comparator 92 has an input coupled to the junction94 between current source 88 and transistor 82 of the current mirror 78,so that the current comparator changes state and provides a Set (S)input to the latch 38 on line 36 when the current in current mirrortransistor 82 exceeds the value n•I_(REF) of the current source 88. Thisin turn will generate an overvoltage protection circuit output signalV_(OVP) from the Q output of the latch 38 at terminal 42, thus disablingswitching transistor 64 and reducing the overvoltage condition. When theovervoltage condition ceases, the latch 38 is reset by a signalV_(RESET) at Reset (R) input 40 of the latch, in the same manner as theprior-art circuit of FIG. 1.

The current source 88 provides a current of n•I_(REF), where I_(REF) isa selected reference current value, with resistor 74 being set such thatI_(SENSE) is equal to I_(REF) when the voltage at junction 72 is equalto its nominal operating value. Since current mirror 78 is selected tohave a 1:1 current ratio, the current in MOSFET 82 will be equal toI_(SENSE), which in the nominal case is equal to I_(REF). Thus, byselecting the parameter n to be equal to a value at which overvoltagedetection and protection is to occur, a desired overvoltage protectionlevel can be achieved. Thus, for example, if n is set equal to 1.26, theovervoltage protection circuit will trigger when the voltage level atjunction 72 (which is proportional to the output voltage V_(DC)) risesabove its nominal value by 26 percent. The current source 88, shownschematically in FIG. 2 by a current-source symbol, may be implementedby conventional means, such as a current mirror having its inputconnected to a source of reference current I_(REF), and a current mirrorratio of 1:n.

In operation, the resistance of variable resistor 74 will set such thatI_(SENSE) is equal to I_(REF) under nominal operating conditions. Thisinitial adjustment not only sets the nominal operating conditions of thecircuit, but also serves to compensate for voltage variations in aparticular circuit due to component tolerances, such as the tolerancesin the transformer winding ratios, which can vary by as much as 30percent, thus causing similar variations in voltages among circuitshaving nominally identical circuit configurations. In this manner, thedrawback of prior-art circuits, such as the one shown in FIG. 1, thatvoltage variations due to component tolerances can lead to substantialvariations in the level of overvoltage that will trigger the overvoltageprotection circuit, is eliminated. Additionally, since the input to theovervoltage protection circuit of the invention is taken directly from atransformer winding, at junction 72, rather than from across capacitor24 as in the prior-art, an overvoltage condition can be detected muchmore rapidly, since the substantial time delay necessary to charge thecapacitor 24 has been eliminated. Thus, overvoltage protection circuitsin accordance with the present invention are both faster and moreaccurate than prior-art circuits.

While the invention has been particularly shown and described withreference to several preferred embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetail may be made without departing from the spirit or scope of theinvention.

What is claimed is:
 1. An overvoltage protection circuit, for aswitched-mode power supply of the type having a transformer with aprimary winding, an auxiliary winding, and a secondary winding, and aswitching transistor coupled in series with the primary winding,characterized in that the overvoltage protection circuit comprises asensing resistor having a first terminal directly connected to saidauxiliary winding for generating a current in said sensing resistorproportional to the voltage across said auxiliary winding and a secondterminal, a current mirror, said second resistor terminal being coupledto an input of said current mirror, an overvoltage reference currentsource coupled to an output of said current mirror, a current comparatorfor comparing a reflected current from said current mirror proportionalto said sensing current in said resistor to an overvoltage referencecurrent generated by said overvoltage reference current source, andcircuit means for generating a signal to turn off said switchingtransistor when said reflected current exceeds said overvoltagereference current.
 2. An overvoltage protection circuit as in claim 1,characterized in that said sensing resistor comprises a variableresistor.
 3. An overvoltage protection circuit as in claim 1,characterized in that said current mirror comprises a delay circuit. 4.An overvoltage protection circuit as in claim 3, wherein said delaycircuit has a time delay of about 500 ms.
 5. An overvoltage protectioncircuit as in claim 1, characterized in that said means for generating asignal to turn off said switching transistor comprises a latch circuit.6. An overvoltage protection circuit as in claim 1, characterized inthat said overvoltage reference current source provides an overvoltagereference current which exceeds a nominal current in said sensingresistor by a selected percentage approximately equal to the percentageabove a nominal voltage of said switched-mode power supply at which saidswitching transistor is to be turned off.
 7. An overvoltage protectioncircuit as in claim 6, wherein said selected percentage is about 26percent.